FIG. 1A is a diagram of a magnetic tunnel device (MTJ) 5 of the prior art. The magnetic tunnel junction 5 is a stack that has a tunnel barrier insulating layer 7 sandwiched between a storage or free magnetic layer 6 and a reference or pinned magnetic layer 8. The storage or free magnetic layer 6 can orient its magnetization 10 and 11 in the desired direction. The reference or pinned magnetic layer 18 has a magnetization 12 direction that is fixed.
In the MTJ 5, the evanescent transmission of the electrons through the tunnel barrier insulating layer 7 determines the resistance of the MTJ 5. In the pinned the free magnetic layer 16 and the magnetic layer 8, the electric current consists of two partial currents, each with either spin-up 10 or spin-down electrons 11. In the tunneling process in which electron spin is conserved, the tunneling conductance depends on whether the magnetizations 10, 11, and 12 of the pinned magnetic layer 8 and the free magnetic layer 6 are parallel 10 and 12 or antiparallel 11 and 12.
FIG. 1B is a diagram of a magnetic electrical test apparatus for determining the electrical properties of a magnetic tunnel junction device of the prior art. The magnetic electrical test apparatus includes a device under test (DUT) holder 15. The DUT holder 15 has a first DUT contact 16 and a second DUT contact 17 between which the MTJ DUT 5 is placed for testing. A ground reference contact 18 is placed on the holder 10 to provide the ground reference for the attached test head electronics 20. The test head electronics 20 has an arbitrary word generator 25 that has an output connected to a first terminal of a first termination resistor 30.
A reference terminal of the arbitrary word generator 25 is connected to a second terminal of the first termination resistor 30. The second terminal of the first termination resistor is connected to the ground reference contact 18 of the DUT holder 15. The ground reference contact 18 is connected to a second terminal of the second termination resistor 35 and thus to a reference terminal of the digitizer 40.
An input terminal of the arbitrary word generator 25 is connected to a computer system 45 that generates a digital word 50. The digital word 50 is applied to the arbitrary word generator 50 to initiate the generation of the stimulus signal that is transferred from the arbitrary word generator 50 to the first terminal of the first termination resistor 30. The first terminal of the first termination resistor 30 is connected to the first DUT contact 16 and thus to one of the magnetic layers 6 or 8 of the MTJ DUT 5.
The stimulus signal is transferred through the MTJ DUT 5 to a second DUT contact 17 and thus to a first contact of a second termination resistor 35 as the MTJ DUT 5 response signal that is the voltage level developed across the second termination resistor 35. The first contact of the second termination resistor is connected to an input terminal of a digitizer circuit 40 that records and digitizes the response signal as a digital word 55. The digital word 55 is transferred to the computer 45 for further processing. The value of second termination resistor 35 is known and the measured magnitude of the voltage developed across the second termination resistor 35 is proportional to the current flowing through the MTJ DUT 5. When the magnitude of the output voltage of the arbitrary word generator 50 as applied to the first terminal of the first termination resistor 30 is measured, the resistance of the MTJ DUT 5 can be inferred.
FIGS. 2A and 2B are plots of a stimulus waveform applied to and a response waveform from the MTJ DUT 5 of FIG. 1A in the magnetic electrical test apparatus of FIG. 1B. One electrical test for the MTJ DUT 5 consists of applying a voltage pulse to the first terminal of the first termination resistor 30 and recording the voltage present at the first terminal of the second termination resistor 35 for determining the resistance state after the pulse to assess the effect of the pulse.
In FIG. 2A, a stimulus waveform is generated in the arbitrary word generator 25 and applied to the first terminal of the first termination resistor 30 and thus to one of the magnetic layers 6 or 8 of the MTJ DUT 5. The signal consists of a stimulus write pulse 60 of high voltage amplitude from approximately 50 mv to approximately 2000 mv for a duration of from approximately 0.5 ns to approximately 2000 ns. The stimulus write pulse 60 is followed by a first stimulus read pulse 65 and a stimulus read pulse 70. The first stimulus read pulse 65 is a negative polarity having an amplitude of from approximately 10 mv to approximately 200 mv for a duration of from approximately 0.5 ns to approximately 100 ns. The second stimulus read pulse 70 is a positive polarity having an amplitude of from approximately 10 mv to approximately 200 mv for a duration of from approximately 0.5 ns to approximately 100 ns.
In FIG. 2B, a response waveform is received from the one of the magnetic layers 6 or 8 of the MTJ DUT 5 and applied to the contact 17 and thus the first terminal of the second termination resistor 35. The signal consists of the response write pulse 75 of a higher voltage amplitude from approximately 0.125 mv to approximately 5 mv for a duration of from approximately 0.5 ns to approximately 2000 ns. The response write pulse 75 is followed by a first response read pulse 80 and a second response read pulse 85. The first response read pulse 80 is a negative polarity having an amplitude of from approximately 0.025 mv to approximately 0.5 mv for a duration of from approximately 0.5 ns to approximately 100 ns. The second response read pulse 85 is a positive polarity having an amplitude of from approximately 0.025 mv to approximately 0.5 mv for a duration of from approximately 0.5 ns to approximately 100 ns. It will be noted that the amplitude of the response signal of FIG. 2B is much lower than the stimulus signal of FIG. 2A as the voltage drop across the second termination resistor 35 that is usually a 50 ohm resistor is much lower than across the MTJ DUT 5 that has a resistance of approximately 20 KΩ.
The resistance state of the MTJ DUT 5 is evaluated by converting the response voltage measurement acquired and digitized by the digitizer circuit 40 and transmitted as the digital word 55 to the computer 45. The computer 45 calculates the resistance of the MTJ DUT 5 since the resistance of the second termination resistor 35 and the applied voltage of the stimulus read pulses 65 and 70 is known. The advantages of having two response pulses 65 and 70 of opposing amplitudes is the ease of removal of any voltage offset present in the testing apparatus (from contacts or amplifier offsets). The offset is removed by calculating the average current during both stimulus read pulses 65 and 70. In current testing apparatus, the computer system 45 that generates the stimulus digital word 50 that is applied to the arbitrary word generator 50 through a universal serial bus (USB) or a General Purpose Interface Bus (GPIB). The digitizer circuit 40 transfers the response digital word 55 to the computer 40 through a PCI eXtensions for Instrumentation (PXI) interface.
One drawback of the magnetic electrical test apparatus of the prior art as shown in FIG. 1B has relatively slow execution time because of the communication handshaking between the computer and digitizer. While a typical stimulation waveform as shown in FIG. 2A may be less than 100 ns, it would take a few milliseconds to get the data to the computer for processing and the computer 40 would take additional 100's of microseconds to process the data. The test patterns executed by the magnetic electrical test apparatus of the prior art may include series of stimulus waveforms generated by the arbitrary waveform generator 25 that include as many as a few thousand waveform patterns. These waveforms patterns must be acquired by the digitizer circuit 40 and digitized into the response digital waveform 55. The digitizer circuit 40 must format the digitized data to conform with the protocol of the PXI interface (or other equivalent interface). While executing test patterns with a larger number of waveforms will minimize the handshaking delay between the digitizer circuit 40, the processing of the digitized waveforms will increase the measurement time overhead and increase the amount of computer memory to store the data of the series of waveforms. For instance, a series of 1,000 waveforms having a pulse width of 100 ns as acquired at a sampling rate of 1 Ghz requires 100,000 data points. If there is a decision making process based on the data recorded during the acquisition by the digitizer circuit 40, an additional overhead of approximately 10,000 times the actual stimulation waveform pulse width may be required.